In general, a cellular radio communication system enables one or more network operators to manage respective distinct networks. When there are several networks, a single zone can be covered by the networks of distinct operators.
Each network is constituted by geographical cells through which mobile stations travel. Each cell of a given network is associated with a base station via which those mobile stations which are to be found in the cell and which subscribe with the operator managing the given network make calls.
Within the coverage zone of its network, each operator transmits a pilot data channel supplying each mobile station with pilot data enabling it, specifically, to log-on to the network, once it has decoded various items of system data relating in particular to the identity of the operator supplying coverage, the services available, which carriers to use, etc.
The quality of network coverage from the point of view of the pilot data channel can be assessed on the basis of the following criteria, in particular:
the time required for a mobile station to log-on initially with the network; PA1 the total spectrum required for a network; PA1 compatibility with future changes to the network and/or to services; PA1 the outgoing data rate; and PA1 transmission times. PA1 each cell of a given network being associated with a respective base station through which it can communicate with those of said mobile stations that are to be found in said cell and that are subscribers with the operator managing said given cell, PA1 each operator transmitting in its network a pilot data channel supplying each mobile station with pilot data enabling it, in particular, to log-on with said network, PA1 the system comprising: PA1 a super-network made up of geographical super-cells each associated with a super-basestation, each super-basestation having means for transmitting a data signal carrying the pilot data channel of the, or each, of the operators, and each mobile station comprising means for receiving and processing said data signal so as to extract therefrom the pilot data channel of the operator with which it is a subscriber; and PA1 means for multiplexing the pilot data channels of the various operators on said data signal. PA1 data concerning the identity of said operator; PA1 data concerning the presence or absence of said operator in the super-cell associated with said given super-basestation; PA1 data concerning channel allocation in the network of said operator; PA1 a data on the identity of the localization zone corresponding to said given super-cell; PA1 data concerning the services available in the network of said operator; and PA1 data concerning the carrier(s), BCCH and/or other carrier, used in the cell(s) corresponding to said given super-cell.
Traditionally, over its entire network, each operator reserves a plurality of radio carriers for transmitting its pilot data channel. Each cell has its own pilot data channel which is carried by a particular carrier known as the BCCH carrier, which carrier is transmitted at constant power by the base station associated with the cell.
To log-on initially with the operator of its network via one of the base stations of the network, a mobile station begins by scanning all of the radio carriers in the allocated spectrum in order to identify, amongst those carriers which are BCCH carriers, the carrier that it receives with the greatest power. It is assumed that the BCCH carrier received at greatest power is the carrier transmitted by the base station associated with the cell in which the mobile station is to be found. Thereafter the mobile station reads the pilot data (in particular the data that will enable it to access the network) as supplied by the pilot channel carried by the BCCH carrier.
In this way, in GSM, the mobile station may have to scan 125 existing carriers and to select the six carriers that are received with greatest power. Then it verifies that the first of the six selected carriers is a BCCH carrier by looking for pilot data of the kind transmitted by BCCH carriers only. If the first selected carrier is not a BCCH carrier, the mobile station moves onto the next selected station and repeats the same operation until it has found a BCCH carrier and the pilot data it carries.
Finally, on the basis of the pilot data read, it can request access on the random access channel (RACH) in order to log-on to the network of its operator.
That present technique for transmitting the pilot data channel of each operator suffers from several drawbacks.
Firstly, because of the need to scan through all possible carriers, the time required by a mobile station to access the network is relatively lengthy.
Another drawback is that each operator, independently of the other operators, must, both on a microcell basis and on a macrocell basis, decide on transmission levels and on reuse schemes within the various cells for the set of BCCH carriers that have been allocated thereto. Implementing this feature at operator level makes future changes difficult to implement.
Finally, coverage quality at the limits of cells is not satisfactory.